In the study by Han et al. [38
] SATB1 expression was found to be greatest in all poorly differentiated infiltrating ductal carcinomas with only low-level expression in some moderately differentiated tumours and complete absence in adjacent normal tissue. Expression was not found to be restricted to late stage disease, but already present in a subset of early primary tumours without lymph node metastasis. SATB1 was found to be of high prognostic significance, independent of the lymph node status and nuclear staining was associated with shorter OS [38
]. In keeping with these results, we also found significantly higher levels of SATB1 mRNA in the breast cancer specimens compared to the background tissue and significant association with poor prognostic parameters including, increasing tumour grade, TNM stage and NPI. Although we observed a trend for tumours with higher SATB1 mRNA expression levels to be associated with shorter OS times, this failed to reach statistical significance. Similarly, significantly higher levels of SATB2 mRNA were found in the breast cancer specimens compared to the background tissue and expression was significantly associated with increasing tumour grade. In contrast to SATB1, tumours classified as having higher transcript levels of SATB2 were significantly associated with a poorer OS.
In-vitro and in-vivo studies have demonstrated that SATB1 can induce the malignant and metastatic phenotype in breast cells, through widespread alteration of gene expression profiles [38
]. In particular, targeted up-regulation has been demonstrated in poor prognosis genes associated with proliferation, metastasis, angiogenesis, degradation of the extra-cellular matrix and tumour invasion, such as metastasin (S100A4), VEGFB, matrix metalloproteases 2, 3 and 9, TGFB1, endothelial growth factor receptor and epidermal growth factor subfamily members, including HER-2/NEU. Furthermore, specific down-regulation of several tumour suppressor genes including BRMS1, KAI1, NME1, and KISS1 has been reported [38
]. In our study, correlation between SABT1 mRNA expression and the downstream regulated genes IL-4 and MAF-1 was identified. IL-4 has recently been implicated in the resistance to cell death observed in epithelial tumours, including breast cancer. IL-4 appears to function as an autocrine survival factor by amplifying expression of anti-apoptotic proteins and preventing therapy related cell death. IL-4 blockade has been shown to decrease growth rate and sensitize cells to chemotherapy [46
]. MAF-1 has been demonstrated to be a key regulator of RNA polymerase (pol) III which produces essential components of the biosynthetic machinery and is therefore a key determinant of cell growth and proliferation [47
]. MAF-1 has also been implicated in regulation of the transformation state of cells [49
]. Hence, expression of SATB1 could represent a singular event with profound implications for tumourigenesis and metastasis in human breast cancer. This would be consistent with the emerging interplay between epigenetics, chromatin remodelling and cancer [50
]. Tumourigenesis and metastasis represent key stages in the development and progression of human cancer. Both are multi-step processes where genetic alterations are associated with characteristic changes in phenotype. Acquisition of metastatic competence is the prelude to widespread dissemination, resulting in the conversion of a local pathology into a systemic disease with associated mortality. [53
]. How cells are initiated along a pathway to malignancy and how malignant cells gain metastatic potential is not well understood. Conventionally, cells are believed to evolve through sequential and additive genetic changes resulting in 'gain of function' and metastatic capacity. However, breast carcinomas with known clinical outcomes have recently been associated with characteristic genetic profiles [55
]. Hence, some cells within the primary tumour may already have the requisite gene expression pattern. The contemporary viewpoint is that such expression profiles may be simultaneously acquired through specific master changes in global genomic organization [38
]. In keeping with this, SATB1 can bind to numerous genomic sites, influencing chromatin organization and orchestrating transcription of multiple genes.
The functional relevance of genomic organization, particularly towards transcription, requires the reconciliation of two distinct paradigms, one suggesting that genomic organization merely reflects nuclear processes including transcription, and the other proposing that dynamic genome organization plays a decisive role in its function [59
]. SATB1 represents a novel class of transcription regulator and has been frequently referred to as a genome organiser. SATB1 appears to serve a dual purpose, structurally as a component of chromatin architecture and functionally as a transcription factor [38
]. Chromatin remodelling is emerging as an important effector mechanism of epigenetic regulation. The role of SATB1 is therefore instrumental to the relationship between higher order chromatin organization and global transcriptional regulation [60
]. SATB1 is a MAR binding protein involved in targeting chromatin remodelling in a tissue-specific manner at specific transcriptionally active chromosomal sites. It interacts with AT-rich sequence motifs within core unwinding elements that become unpaired under conditions of torsional/superhelical stress. Such base-unpairing regions (BURs) augment the potential for enhancers to act over large distances [10
]. The protein selectively tethers multiple genomic loci and is localized to characteristic 'cage-like' three dimensional protein scaffold that anchors loops of chromatin, forming a dynamic chromatin 'loopscape' [20
]. By acting as a 'docking site' it recruits chromatin remodelling complexes and regulates histone modifications and nucleosome positioning over long stretches of DNA [20
The regulation of the activity of MAR-binding proteins remains poorly understood. Post-translational modification of SATB1 may provide a subtle refining mechanism for regulation of chromatin architecture and gene transcription [60
]. In this way, SATB1 may act as repressor or activator depending upon the physiological context [18
]. At an early stage of apoptosis, SATB1 is irreversibly inactivated by caspase 6 cleavage, resulting in rapid dissociation from chromatin and MARs [70
]. SATB1 is also one of the few nuclear proteins harbouring the PDZ protein-protein interaction domain, permitting interaction with many nuclear proteins [20
]. Intriguingly, SATB1 does not contain a classical nuclear localization signal and a novel N-terminus sequence motif has been found to mediate nuclear localization [29
]. In contrast to SATB1, SATB2 can be reversibly and dynamically modified by SUMOylation (covalent conjugation of the small ubiquitin-related modifier), which appears to modulate its activity as a transcription factor. SUMOylation is also involved in targeting SATB2 to the nuclear periphery [10
]. The structural similarity between SATB1 and SATB2 implies analogous roles in transcriptional regulation. Furthermore, these proteins are highly conserved across species [36
]. Further studies are required to determine the genome-wide targets of SATB1 and the multitude of interacting factors recruited. Furthermore, the process underlying tissue specificity of regulated genes and the mechanisms of transcriptional up- or down-regulation will need to be fully elucidated to optimise the impact of any potential therapeutic manipulation strategies [74
]. Chromatin organizers are unlikely to be unique to breast cancer pathogenesis and study of other human cancers is warranted.
Limitations of the present study included the use of background parenchyma from breast cancer patients to provide 'normal tissue' for comparison. Ideally, such material should be derived from patients without breast cancer in order to avoid any 'field change' which may exist within cancer bearing tissues. Although the sample size and follow-up period were substantial, it is possible that a larger cohort may have influenced several results which approached, but failed to reach, statistical significance. Finally, in addition to the measurement of mRNA transcript levels, quantitative analysis of protein expression should be undertaken to ensure concordance.